The present invention relates to a tensioner for motorcycles and automobiles, more particularly to a tensioner for tensioning a drive chain or belt for driving a camshaft of an engine.
A conventional tensioner such as a belt tensioner or a chain tensioner is used to apply constant tension on a belt or chain even when it looses predetermined tension due to elongation or slackening caused by wear or friction. For this reason it is preferable that the tensioner may be kept advanced at all times in a tensioning direction and may have such a characteristic as a rigid object against a counterforce in an opposite direction. FIG. 3 shows a vertical section of a conventional tensioner.
The tensioner shown in FIG. 3 comprises a casing 1 with a housing 1a formed in an axial direction of the tensioner, a rotational cylinder 2 inserted rotatably in the housing, a tension rod 3 engaged with a threaded portion of the rotational cylinder 2, a torsion spring 4 inserted externally on the rotational cylinder 2. One end 4a of the tension spring 4 is inserted in a slit 2a formed on the rear end of the rotational cylinder 2 and the other end, a hook 4b is inserted in a long groove 1b formed in the front portion of the housing 1a for driving the rotational cylinder 2. A seal bolt 6 is screwed on the rear end of the casing 1. The top of the tension rod 3 is covered with a cap 8 fixed with a spring pin 7, whereby a chain or belt is tensioned directly or indirectly to keep constant tension. The tension rod 3 is externally round and diametrically has two plain surfaces. And it is inserted freely in a corresponding slot of same shape formed on a bearing 9 so as not to turn, whereby the rotating force of the rotational cylinder 2 can be converted in urging force.
This type of tensioner can be applied a predetermined torsional torque by using a flat driver through a threaded hole on the rear end of the casing after mounting the components in the housing and then the threaded hole can be hermetically closed with an "O" ring 5 and bolt 6. Thus the tensioner can be mounted, for example on the outer wall of equipment such as an engine body by inserting the front portion of the casing so that the cap of tension rod may keep constant tension on chain or belt by means of restoring force of the torsion spring.
It works satisfactorily when mounted. However, it is not easy to mount the components such as tension rod, rotational cylinder and torsion spring in the housing. Therefore a ring groove of small diameter is formed between the cylinder portion and threaded portion of the rotational cylinder so that a flat washer may be freely mounted so as to facilitate the mounting of a collar 11 between the bearing 9 and a flat washer 10. In this way the movement of the rotational cylinder 2, tension rod 3 and the torsion spring 4 in the axial direction can be restricted by the flat washer 10, collar 11, bearing 9 and a snap ring 12.
Hereinafter the problems to be solved by the present invention will be explained. In the conventional belt or chain tensioner, when the torsion spring 4 is wound to get torsional torque, the hook 4b penetrates into the gap between the flat washer and the rotational cylinder 2 sometimes impeding the normal function of the torsion spring 4 or impeding the normal rotation of the rotational cylinder 2. It is because the torsion spring 4 extends in the axial direction when wound and at the same time occurs eccentrical movement causing such problems aforementioned. Then it is effective to make the gap between the collar 11 and the flat washer 10 as small as possible, but it is very hard to reduce this gap ignoring the diameter of wire material.